(1) Field of the Invention
This invention relates to motion transmitting mechanisms, and more particularly to flexsplines for harmonic drive type transmissions.
(2) Prior Art
In harmonic drive transmissions, as may be seen for instance in U.S. Pat. No. 3,789,700 to Cotreau et al, or as initially disclosed in U.S. Pat. No. 2,906,143 to Musser, concentric gears are interengaged at spaced circumferential locations. These locations are advanced by a wave of radial deflection in one of the gearing elements commonly termed a flexspline which reacts on a rigid circular spline. The radial deflection in the flexspline may be caused by a wave generator, generally of ellipsoidal shape, rotating therewithin, the flexspline therefore, generally having an ellipsoidal shape therein, with a major axis arranged across the lobes of the wave generator.
The flexspline has had essentially the same configuration for the past decade, i.e. a thin tubular element adjacent a toothed portion arranged around the open end of the flexspline. This may be seen for instance, in U.S. Pat. No. 3,161,082 to Musser. Other configurations of flexsplines may be seen for example in U.S. Pat. Nos. 3,139,770 to Musser; 3,147,640 to Musser; and 3,311,766 to Scott. Even during development of these patents, it has been known that a torque load applied to the flexspline by the circular spline tends to skew the deflection wave imposed in it by the wave generator. The principle deflection stress in the skewed flexspline is surprisingly not at the major axis of the flexspline where the gear teeth are fully interengaged between the rigid circular spine and the flexspline, but at the location in the flexspline where the maximum change in the radius of curvature occurs, which is just behind the major axis.
The amount of skewing is dependant upon the clearance between the wave generator and the bore of the flexspline along the minor axis, with a loose fitting wave generator, such as that shown in the above-mentioned U.S. Pat. No. 3,161,082 to Musser, allowing the most skewing to occur. In this situation, as a torque load is being applied to the flexspline via the circular spline, the flexspline attempts to wrap itself around the wave generator absent any support in the area of the minor axis, causing skewing of the deflection wave within the flexspline. This uncontrolled deflectional shape in the flexspline introduces rotational displacement which adds additional windup into the system, and increases the deflection stress which is undesirable.
It is an object of the present invention, to provide a flexspline which minimizes skewness therein from standard ellipsoidally shaped wave generators, as well as the double eccentric wave generators.